1. Field of the Invention
The present invention relates to an improved arrangement of magnets in the cathode assembly of a magnetron sputtering apparatus.
2. Description of the Prior Art
With a magnetron sputtering apparatus for sputtering processing by stationary magnets, the surface of the target is eroded locally intensely; therefore, the utilization efficiency is low. To erode uniformly across the target, it has been proposed that the magnets be designed to be movable during sputtering as described in Japanese Patent Laid-Open Publication (Kokai) Sho 53-7586 (1978). One of the modifications so far developed for moving the magnets is to rotate them.
FIGS. 4A-4C show a magnetron sputtering apparatus described in U.S. Pat. No. 5,047,130 in which the magnets are rotated. FIG. 4A is a plan view of the cathode electrode assembly of the magnetron sputtering apparatus, in which permanent magnets 6N and 6S have north (N) and south (S) surface poles, respectively, are arranged as shown. FIG. 4B is a sectional view as taken on line III--III in FIG. 4A. The cathode assembly provided within the vacuum chamber (not shown) of the magnetron sputtering apparatus comprises a target 2, a cathode 4 to place the target thereon, a yoke 6 facing the back side of the cathode 4, on which permanent magnets 6N and 6S are mounted, a yoke holder 21 and a rotary shaft 8. When the shaft 8 rotates, the permanent magnets 6N and 6S turn with the yoke 6 and the yoke holder 21 being rotated. The magnetic lines of force 30 produced by the magnets 6N and 6S pass through the target 2 and form loops that are closed above the target. As shown in FIG. 4B, a substrate 60 mounted on a substrate holding electrode 62 (by mounting means not shown) is provided above the target.
When argon (At) gas is supplied into the vacuum chamber and a DC voltage applied between the cathode electrode 4 and the substrate holding electrode 62 from a power supply unit (not shown), electrons are produced that revolve spirally around the magnetic lines of force 30. As a result, a magnetron discharge is generated. By the generated magnetron discharge, the surface of the target 2 is sputtered, allowing a thin film of the target material to be deposited on the surface of the substrate 60.
As one can also see from FIG. 4A, the magnet 6S is provided in the back side of the central portion 100 of the target 2 and has a south (S) surface pole. The magnet 6N is located around a looped region EA surrounding the magnet 6S and has a north (N) surface pole. If sputtering is performed without rotating the yoke 6 (i.e., the magnets are held stationary), the surface of the target 2 will be eroded in a region of a shape that is substantially identical to the looped region EA.
FIG. 4C is a sectional view as taken on line III--III in FIG. 4A to show the depth profile of a target 2 that was eroded by sputtering while the yoke 6 is rotated (the dimension of the target in the direction of thickness is exaggerated in FIG. 4C). As can be seen from FIG. 4C, the target 2 was only barely sputtered in the central portion 100. The fine dashed line in FIG. 4C delineates the depth profile of a target 2 that was eroded by sputtering while the yoke 6 is held stationary. The most deeply eroded area E occurs depicting a loop that is delineated by a closed curve EA (as indicated by the dashed line in FIG. 4A) on the surface of the target. If the yoke 6 is stationary, a non-eroded area will occur on the surface of the target. The erosion area of the target 2 is expanded by rotating the yoke 6. Japanese Utility Model Laid-Open Publication (Kokai) Hei 2-99962 is similar to the cathode assembly of U.S. Pat. No. 5,047,130 in that the eroded area is expanded by rotating the yoke. Furthermore, U.S. Pat. No. 5,182,003 discloses a yoke having a magnet arrangement as shown in FIG. 12, but the yoke does not itself rotate. In this arrangement, even if the yoke is rotated, a noneroded area will occur on the surface of the target.
With the conventional cathode assembly described above, the target 2 is barely sputtered in the central portion 100. If some part of the target remains unsputtered, the sputtered particles will be deposited on that unsputtered area to form a thin film. If the deposited film grows to a thickness that develops internal stress, the film will shed off or crack, causing particulate contamination.
The recent tendency in the field of semiconductor fabrication is to use a silicon (Si) substrate of larger diameter for VLSIs and the heretofore common 6-inch substrates are increasingly being replaced by 8-inch or even 10-inch substrates. This has accordingly created the need to increase the diameter of the target. One of the problems with the conventional arrangement of magnets shown in FIG. 4A is that as the distance between magnets 6N and 6S increases, the plasma density will decrease, making it difficult to produce a thin film at high speed. This is because given the coercive force of two magnets with N and S poles, the flux density of magnetic lines of force 30 passing per unit area (the elliptic hatched areas in FIGS. 6A and 6B) between the two magnets will decrease as the distance between the two magnets increases. Magnetron discharge is usually created by applying an electric field (electric lines of force) E crossing a magnetic field (magnetic lines of force) at a right angle. Given the intensity of the applied electric field (electric lines of force), the density of plasma generated by the magnetron discharge is proportional to the flux density of magnetic lines of force 30 passing per a unit area (the elliptic hatched areas in FIGS. 6A and 6B) parallel to the applied electric field (electric lines of force). Therefore, given the coercive force of the magnets producing the magnetic field (magnetic lines of force) and also given the intensity of the electric field (electric lines of force) crossing the magnetic field (magnetic lines of force) at a right angle, the plasma density of the magnetron discharge will be in inverse proportion to the distance between the two magnets. In other words, the plasma density of the magnetron discharge will decrease with the increasing distance between the magnets.